4-1 Moment Tensor – Applications 4-2 General Remarks on Applications to Structures

1. Short Course 101-1111-00L:Fundamentals and Applications of Acoustic Emission October 25, 2007 4-1 Moment Tensor – Applications 4-2 General Remarks on Applications to Structures 4-3 Concrete – Fresh and Hardened

2. 4-1 Moment Tensor – Applications October 25, 2007 (1/3) • Damage Mechanics • Error Estimation • Visualization

3. Introduction • Kinematical information of crack motions is recovered by the SiGMA analysis. • Some applications are stated. The development of the SiGMA procedure is closely associated with error estimation. • In addition, the moment tensor is mathematically related with damage mechanics.

4. Damage Mechanics • Trace component : Mkk = (3l+2m)lknkDV. • A damage parameter in damage mechanics [Kachanov 1980], D = ∫ni(binj)nj dS = DVlknk = Mkk/(3l+2m) Further the crack volume is obtained, • DV = Mkk/[(3l+2m)lknk].

5. Results of the center-notched specimen (type CC), and the off-center notched specimen (type OC) under bending

6. Error Estimation • Estimation of errors in the moment tensor analysis is fairly difficult. In the analysis of hydro-fracturing, the conditioning numbers were applied. • Then, error estimation was conducted, assuming the errors. As a result, it is found that the error estimation has been a really complicated task, because the errors are dependent on a spatial relation between the source location and the observation points. • Consequently, a post analysis is proposed [Ohtsu 2000], where AE waves to be detected at the observation points are synthesized from the source location and the moment tensor theoretically. The simulation analysis of theoretical AE waves is readily available.

7. Leakage test of PMMA plate with a slit • 102 AE events with detectable first arrivals were analyzed by the two-dimensional SiGMA analysis. Results are classified by the shear ratio X (%). • It is clearly found that almost 60 % events have the shear ratios over 60 %. • It was, however, realized that AE sources were distributed widely on the specimen, not concentrated around the slit. • Determination of both the amplitude of the first motion and the arrival time was actually not an easy task. This implies that some errors are unwillingly contained in results of the SiGMA analysis.

9. Post-Analysis • AE waveforms at sensor locations were synthesized in an infinite space, taking into the source location and the moment tensor components. The reflection coefficient was taken into consideration to simulate the waveforms. The SiGMA procedure was applied to synthetic waveform set as the post analysis.

10. Following the post analysis, events of the shear ratio less than 10% different from those of the SiGMA analysis were selected as reliable solutions.

11. Visualization • Visualization procedure is developed by using VRML (Virtual Reality Modeling Language). • By applying the SiGMA code, AE events are displayed at their locations with symbols. In the previous results, a tensile crack is denoted by arrow symbol, of which direction is identical to that of crack opening. A shear crack is denoted by cross symbol, of which two directions correspond to two vectors l and n.

12. VRML • Although classification of cracks was readily made, crack orientation was not easily recognized. This was because two-dimensional projection was adopted for illustration but analyzed results are inherently suitable for three-dimensional visualization. In this respect, VRML is introduced.

13. Crack kinematics in the bending test of reinforced concrete beam

16. Crack kinematics in the bending test of reinforced concrete beam • Diagonal-shear failure at the shear span

17. Loading stages

18. 1st stageLight Wave 3D Software

19. 2nd stage

20. 3rd stage

21. Comparison with final failure

22. Remarks • Nucleation of cracks can be kinematically analyzed by the moment analysis. Applying the SiGMA code. • For the two-dimensional soltution, in-plane motions of AE waves are treated. The reliable solutions are selected by the post analysis. • Because visualization of results is desirable, three-dimensional visualization procedure is developed.

23. 4-2 General Remarks onApplications to StructuresOctober 25, 2007 (2/3) AE Inspection Basics of Measurement Elimination of Noises Set-Up of AE System and Measurement

24. Introduction AE techniques have been applied to a variety of structures and infrastructures in the field of civil engineering. Successful applications are concrete, rock, wood, superstructures of buildings and bridges, and substructures including railway structures. A few codes have been established on concrete structures only in Japan [NDIS 2421 2000] and [JCMS-III 2003]. This is because concrete structures are known to be no longer maintenance-free.

25. Codes and Standards When the techniques are going to be applied to existing structures or local members, inspection procedure shall be based on the codes and standards. In other kinds of structures than concrete, in-situ inspection techniques of AE measurement or monitoring are under development for maintenance. Some codes and manuals have been standardized in the governmental institutes and departments.

26. Basics of Measurement Due to damage evolution and deteriorations in the structures, AE events are observed, nucleating microcracks under in-service conditions. An inspection method for active cracks or defects is a target for AE applications in engineering.

27. AE Sensor AE sensors shall be sensitive enough to detect AE signals generated in the target structure, taking acoustic coupling into consideration. Sensitivity calibration of AE sensors shall be performed by employing the standard source or an equivalent piezoelectric sensor. AE sensor also shall be robust enough against temperature changes, moisture conditions and mechanical vibrations in the environments.

28. Standard AE source

29. Coupling Concerning coupling, several kinds of couplants are available. So far there exist no regulated techniques as long-term inspection or durability of couplants are not confirmed yet. This is because AE techniques are still immature for field applications. === Good enough strength and durability===

30. Amplifiers Amplifiers shall be set up as close as possible to AE sensors. The frequency range, which are usually controlled by filters, and shall be determined prior to the measurement.

31. AE parameters count, hit, event, maximum amplitude, energy, rise time, duration, energy-moment, RMS voltage, frequency spectrum, arrival-time difference.

32. Elimination of Noises Elimination of the noises is one of the most concerned aspects in the applications. Usually, it is achieved by simply setting the threshold level over the noise level, or by a band-pass filtering and a post-analysis of the data. Environmental Noise In advance to AE measurement, the noise level shall be estimated on site. Counteract against external noises, such as wind, rain, sunshine shall be made.

33. Separation of Noise In the case that the noises have similar frequency contents and amplitudes to AE signals, or sources of the noises are unknown, characteristics of the noises shall be estimated prior to the measurement. Then, separation of AE signals from the noises shall be made. In this respect, the use of filters is useful after determining the proper frequency range.

34. Post-Analysis AE events detected under traffic passing. the measuring system was turned off in each case of traffic passing

35. Set-Up of AE System A measurement system consists of AE sensor, amplifier, and filter. Total amplification by the pre-amplifier and the main amplifier is usually set from 60 dB to 90 dB. To decrease the noises, a band-pass filter between several kHz and 1 MHz is mostly desirable. The noises should be lower than 20 mV as input voltage after detected by AE sensors.

36. System Setting Elimination of the noises is achieved by simply setting the threshold level over the noise level, or by a band-pass filtering and a post-analysis of the data. In any cases, the averaged amplitude of the noise should be managed to be lower than 10 mV as input.

37. Sensor Array Sensor array is determined from the attenuation properties of AE waves, setting the distance where attenuation during travel is less than 30 dB. In most cases, the distance between the sensor and an AE source is set to be shorter than 1 m, in relation to the attenuation property,

38. Attenuation in concrete Sensor array shall be determined so as to keep the equivalent sensitivities in all the sensors. The frequency range from 20 kHz to 100 kHz is recommended for in situ monitoring of concrete structures. In advance of the test, attenuation properties of the target structure shall be estimated, by employing the standard source or the equivalent.

39. AE sensors are attached at proper locations to cover the target area. The period of the measurement shall be prescribed, depending on the following conditions: (1) Propagation property of AE signals in the target structure (2) Stress distribution in the structure under inspection Sensitivity of AE channels shall be conducted routinely by employing the standard source. Variation within the channels shall be less than 3%. Based on the spatial area to be covered, AE sensors of proper frequency characteristics shall be selected

40. Measurement AE signals shall be detected properly for the period of the measurement. Concerning AE parameters detected, their trend, distribution, correlation, and locations are monitored and measured. In principle, AE tests are conducted under loads which must not cause any damages on functions of the structure during detection and location of active cracks.

41. Tests to be carried out routinely or temporally Under the following loads: Service load lower than the serviceable limit Incremental load lower than the serviceable limit Variable and repeated load during service Relations among AE parameters, at least AE hits, time, and loads shall be analyzed.

42. Remarks Applied fields and structures of AE measurement are still under developing and expansion. This is because set-up conditions of AE measuring systems and target of analyses are different in each case and each site. As a result, it is stated that AE techniques in practical fields are still immature. But, the applicability is already confirmed and successful applications have been extensively reported.

43. 4-3 Concrete – Fresh and HardenedOctober 25, 2007 (3/3) AE Techniques in Concrete Concrete of Early Age Hardened concrete

44. Introduction AE techniques have been extensively studied in concrete engineering for approximately five decades [Ruesch 1959]. They are applied to practical applications [Ohtsu 1987] and are standardized in the code [NDIS2421 2000]. This is because the increase of aging structures and disastrous damages due to recent earthquakes urgently demand for maintenance and retrofit of reinforced concrete structures in service. It results in the need for the development of advanced and effective inspection techniques.

45. Modern AE research in concrete Wells studied the relationship between strain measurement and AE event [Wells 1970]. Studies on fundamentals of AE activity and the effects of mixture proportion were conducted [McCabe et al. 1976, Nielsen & Griffith 1977, Mlakar, Walker et al. 19845]. A frequency analysis and a source location analysis were also reported [Fetis 1976, Niwa, Kobayashi et al. 1978, Reymond, Raharinaivo et al. 1983, Berthelot & Robert 1987, Weiler, Xu et al. 1997].

46. Diagnostic Applications in Concrete Applications to reinforced concrete structures were investigated [Niwa, Kobayashi et al. 1977, Kobayashi, Hawkins et al. 1980]. These studies have resulted in practical applications to monitor micro-cracks in concrete structures and going to be made practical as diagnostic applications[Ohtsu 1988].

47. Concrete of Early Age Nondestructive tests for fresh concrete at early ages have been summarized in the Rilem state of the art report [Reinhardt & Grosse 2005]. The AE applications to concrete of early ages are included in the report. Based on the article, noteworthy examples of the applications are briefly stated.

48. Under Mixing Roller-compacted concrete is developed and extensively applied to construct concrete dams in Japan. In order to control the consistency of fresh concrete, the VC test is carried out.

49. VC test Fresh concrete after mixing is placed in the mold on the vibration table. During the vibration, time is measured by second, which is named “VC value”, until breeding water is observed at the surface of the mold.

50. AE activity under mixing The measurement is neither practical nor easy to estimate concrete properties of all mixes. During mixing, where AE sensor was attached directly to the outer surface of a concrete mixer.